Currently, in an attempt to enhance the performance of thin-film transistors (TFTs), and lower the temperature and costs of the manufacturing process of it, a broad range of materials have been researched and studied for channel layer materials. Examples of the materials include amorphous silicon, polycrystalline silicon, microcrystalline silicon and organic semiconductors.
Oxide semiconductors, which have recently been discovered, are also a prevailing group for the materials. For example, Barquinha et al., J. Non-Cryst. Sol., 352, 1756 (2006) and Yabuta et al., Appl. Phys. Lett., 89, 112123 (2006) disclose a TFT fabricating method using an amorphous In—Zn—O (IZO) thin film and an amorphous In—Ga—Zn—O (IGZO) thin film, respectively, for a channel layer.
TFTs have different threshold voltages depending on the semiconductor material for the channel layer and/or the manufacturing method. Also, the threshold voltage of a TFT varies due to various causes (such as manufacturing process history, temporal change, electrical stress and thermal stress). Here, electrical stress refers to stress occurring upon application of a voltage or current to a semiconductor. Also, thermal stress refers to stress occurring upon heating the area around a semiconductor from the outside, or by Joule heat generated upon application of a current to the semiconductor. These two types of stress may simultaneously be imposed on a TFT during operation.
Even TFTs using the above oxide semiconductors as a channel layer are no exception: Riedl et al., Phys. Stat. Sol., 1, 175 (2007) and Kim et al., International Electron Device Meeting 2006 (IEDM'06), 11-13, 1 (2006) indicate that a threshold voltage variation due to electrical stress or a combination of electrical stress and thermal stress has been observed.
Furthermore, Barquinha et al., J. Non-Cryst. Sol., 352, 1756 (2006) discloses that the threshold voltage of an oxide semiconductor TFT varies upon visible light and ultraviolet light irradiation. Also, for polycrystalline silicon, Japanese Patent Application Laid-Open No. H10-209460 discloses a method for reducing the threshold voltage of a TFT by providing a structure that allows light to enter the channel layer of the TFT.